Metal containers for receiving foods generally have a coating, so as to prevent contact between the filled product and the metal, so that there is no disadvantageous influence on the quality of said product and so as to prevent corrosion to the metal by said product. For producing containers of this type, such as tin cans, use is made of metal sheets which, prior to their deformation or shaping, are provided with a suitable coating. The known epoxy-phenolic lacquers are suitable for this purpose and as a result of their colour tone they are also referred to as gold lacquers. Of late and in particular in conjunction with modern processing and deformation processes (cf. below) more and more PVC-based organosols have been used, which in addition generally contain a phenolic resin, a polyester resin, a diluent, a catalyst, a polymer plasticizer and optionally conventional additives (cf. e.g. German Patent 20 29 629 and European patent application 254 755).
Of the various processing and deformation processes which can be used, within the scope of the present invention particular interest is attached to deep drawing and more especially multi-stage DRD deep drawing (draw and redraw). These processing and deformation processes make increased demands on the sheet metal coating, i.e the coating must not only be as free as possible from pores and cracks and adhere well, but must also have a good drawing property or capacity, so that the afore-mentioned characteristics are also present following deep drawing. Whereas for the production of three-part tin cans use is made of sheet metal thicknesses of approximately 0.19 to 0.25 mm, for DRD deep drawing harder, but thinner metal sheets with a thickness of 0.18 mm and less are used. A suitable material is e.g. tin plate. However, preference is given to metal sheets, which are given an extremely thin chromium surface coating.
The afore-mentioned organosols are particularly suitable for the coating of metal sheets, which are to be shaped or deformed in accordance with the afore-mentioned deep drawing processes. The organosol is applied prior to the deformation of the sheet (normally after a roll coating process) and is then normally baked for 8 to 15 minutes at approximately 175.degree. to 225.degree. C. For this purpose the metal sheet is generally passed through a drying tunnel. In the case of two-sided sheet metal coating, the organosol is firstly applied to one side of the sheet and then baked and subsequently the other side of the sheet is treated in the same way. As a function of the intended use, the coating thicknesses are between 7 and 30 g (dry weight) per m.sup.2. It is important that the coating adheres well, has no pores and cracks and is mechanically stable, so that a coating or lacquer is obtained, which is resistant to corrosion and sterilization and which is not sensitive to mechanical stresses.
Although PVC-based organosols lead to coatings which largely meet the demands of deep drawing processes, there is a considerable need for further improvements to the coating characteristics, i.e. in particular for a further reduction to the porosity and an improvement to the stability with respect to more or less aggressive filled products. Reference is also made in this connection to the prevention of the diffusing of the plasticizer into the filled material. An important disadvantage of coatings based on PVC-containing organosols is that the PVC absorbs colourings like the pink colouring of the shrimps and crabs, so that when emptying a can filled with such a product, an unattractive pink staining can be seen on the internal coating which, although admittedly not impairing the product quality, is certainly an irritant to the user.
Another possibility for the coating of metals for the production of receptacles such as tin cans, tubes and sheet metal containers of all types is the afore-mentioned use of epoxy-phenolic lacquers, which are applied to the sheet metal in the same way as the afore-mentioned organosols and are then baked for 8 to 15 minutes at approximately 175 to 225.degree. C. The resulting coatings have excellent characteristics and in particular a very good chemical stability and therefore resistance to the action of aggressive filled products. However, these lacquers suffer from the disadvantage that resistance to aggressive filled products is not combined with adequate elasticity and that therefore they are not drawable and can consequently not be used for deep drawing processes and especially DRD deep drawing processes.
The problem of the invention is to further improve the coatings and in particular the inside coatings of metal containers for receiving foods and to avoid the afore-mentioned disadvantages of the known coatings.